Voxy 36W project

[tomandrew]

Amongst my American and British styled one-channel and multichannel builds there’s one thing missing: a Vox style EL84 amp. (Well… apart from that Weber 18W TMB thing that’s eating fuses…)

Since Vib/Trem isn’t my thing I’m planning on using an 18W chassis, but adding another couple of power tubes and a bigger set of transformers.

I’m pretty much set on the preamp, but I’m open to suggestions:
Channel 1 would be a straight up Vox normal channel, just a Volume knob, straight to the PI.
Any forum opinions on the choice of coupling cap size? Should I add a bright cap or not?

I’m thinking shared cathode for V1a/V1b.

Channel 2 would be a Top Boost preamp.
I’d be playing around a bit with first coupling cap size, 1nF or 500pF.

I’d be going for classic Vox values for the PI.
Again, coupling caps, both before and after PI?

However, the really undecided stuff:
I’m planning on going diode rectifier and resistor instead of choke.
Bigger screen resistors, would 470R be enough or should I go higher?
EL84 cathode resistor size; two 5W 180R in parallel should keep things reasonable.
Master volume (a simple phase-cancelling one) or half power switch? Separating the EL84’s into pairs with their own cathode resistor/cap makes for an easy implementation, I’m however not sure if I’d ever use the half power setting.
Filter caps; should I go for Vox style 16uF+16uF+8uF+32uF+8uF or go for more capacitance after the rectifier?

Thanks!

[sluckey]

I’m thinking shared cathode for V1a/V1b.

That's a bad idea if V1a/V1b will be cascaded. Risk of positive feedback. Spend another dollar.

[tomandrew]

I’m thinking shared cathode for V1a/V1b.

That's a bad idea if V1a/V1b will be cascaded. Risk of positive feedback. Spend another dollar.

Noted!
I don’t see myself cascading them; I’d probably only use Channel 1 if using an external preamp, but I’ll take your advice into consideration.
Thanks!

[sluckey]

If not cascaded, how will you connect them?

[tomandrew]

If not cascaded, how will you connect them?

Hmm…
Channel 1 uses V1a, has its own plate resistor and coupling cap, vol knob connects to one side of PI.
Channel 2 uses V1b (and V2) with its own plate resistor etc. Channel 2’s treble pot connects to the other side of the PI.
Just like the Vox amps of yore, except I’m not mixing the channels before the PI.

[pompeiisneaks]

Hmm…
Channel 1 uses V1a, has its own plate resistor and coupling cap, vol knob connects to one side of PI.
Channel 2 uses V1b (and V2) with its own plate resistor etc. Channel 2’s treble pot connects to the other side of the PI.
Just like the Vox amps of yore, except I’m not mixing the channels before the PI.

You also need to account for phase, if you have one channel going through one gain phase, and the other going through another, and someone uses both, you have phase cancellation if there are a different number of gain stages on each half that aren't both either even or odd... i.e. channel 1 with 2 gain stages and 2 with 3 is a problem, but... that's only if you either jumper them, or have someone playing into both with 'similar' music, then they can cause phase issues.

~Phil

[pdf64]

Feeding one channel into one LTP grid, and the other into the other grid, will get around that ‘opposing polarity signals’ issue.

[sluckey]

OK, in this case there's no problem connecting cathodes together. But there are no benefits other than saving a few pennies.

You also need to account for phase, if you have one channel going through one gain phase, and the other going through another, and someone uses both, you have phase cancellation if there are a different number of gain stages on each half that aren't both either even or odd... i.e. channel 1 with 2 gain stages and 2 with 3 is a problem, but... that's only if you either jumper them, or have someone playing into both with 'similar' music, then they can cause phase issues.

In this case you will have phase cancellation if you have the SAME number of gain stages in each channel. This is because one channel goes to the inverting input of the LTPPI and the other channel goes through the non-inverting input of the LTPPI

[tomandrew]

I’d be looking at saving some real estate rather than money by sharing the V1 cathode.
Also, it would be more simplistic and true to the original.
And, I have a tendency to build amps with too many switches and modes, this would be a more straightforward and clean build.

Any opinions on the B+ filtering?

[thetragichero]

if you have room for another small bottle, what if you used a pentode for channel one? i have a bare bones voxy build that uses a 12au6 (the pt only has a 12v heater winding, 6au6 would work for your needs) into volume control into long tail pair with the cut control as the only tone control. sweet little amp that may provide something a bit more tonally interesting to mix with the top boost channel and those au6 tubes are cheap as chips (just make sure to get a 7-pin socket and i'd suggest one with a shield)